Riding behind a bike with loud pipes spoils my ride.
I bought a used Harley with V&H short shots on it. They were maybe 30" long .... Thing is, the bike ran terribly with no appreciable back pressure.
Back pressure is always a detriment. The real need for pipes of a certain length is the sound pressure wave propagation. Pipes that are too short (or headers too short on a multi-cylinder engine into a collector) does not run the best. Here's more:
"As the exhaust valve opens, the relatively high cylinder pressure (70 – 90 psi), initiates exhaust blowdown and a large pressure wave travels down the exhaust pipe.....As the exhaust pressure wave arrives at the end of the exhaust pipe, part of the wave is reflected back towards the cylinder as a negative pressure (or vacuum) wave. This negative wave, if timed properly to arrive at the cylinder during the overlap period can help scavenge the residual exhaust gases in the cylinder and also can initiate the flow of intake charge into the cylinder. Since the pressure waves travel at near the speed of sound, the timing of the negative wave can be controlled by the primary pipe length for a particular rpm.....the trick to proper exhaust tuning is to tune an exhaust system is produce a negative wave of the proper strength timed to occur at cylinder overlap."